Fluoroquinolone resistance in clinical isolates of Streptococcus pneumoniae:: Contributions of type II topoisomerase mutations and efflux to levels of resistance

We report on amino acid substitutions in the quinolone resistance-determining region of type II topisomerases and the prevalence of reserpine-inhibited efflux for 70 clinical isolates of S. pneumoniae for which the ciprofloxacin MIC is greater than or equal to4 mug/ml and 28 isolates for which the ciprofloxacin MIC is less than or equal to2 pg/ml. The amino acid substitutions in ParC conferring low-level resistance (MICs, 4 to 8 mug/ml) included Phe, Tyr, and Ala for Ser-79; Asn, Ala, Gly, Tyr, and Val for Asp-83; Asn for Asp-78; and Pro for Ala-115, Isolates with intermediate-level (MICs, 16 to 32 mug/ml) and high-level (MICs, 64 mug/ml) resistance harbored substitutions of Phe and Tyr for Ser-79 or Asn and Ala for Asp-83 in ParC and an additional substitution in GyrA which included either Glu-85-Lys (Gly) or Ser-ll-Phe (Tyr). Glu-15-Lys was found exclusively in isolates with high-level resistance. Efflux contributed primarily to low-level resistance in isolates with or without an amino acid substitution in ParC. The impact of amino acid substitutions in ParE was minimal, and no substitutions in GyrB were identified.